Recently, interest in the energy storage technology has been increased. The effort to research and develop an electrochemical device has been gradually materialized as the application field of the energy storage technology has been expanded to a mobile phone, a camcorder, a notebook PC, and an electric vehicle. The electrochemical device is a field which attracts the most attention in this respect, and in particular, the development of a secondary battery capable of being charged and discharged is the focus of attention.
Among the secondary batteries which are currently applied, the lithium ion battery developed in the early 1990s has been widely used as a power source or portable apparatuses since it was developed in 1991 as a small battery, a light-weight battery, and a large capacity battery. The lithium secondary battery is in the spotlight due to its advantages that the operating voltage is higher and the energy density is far greater as compared to batteries of prior art, such as a Ni-MH battery, a Ni—Cd battery, and a sulfuric acid-Pb battery which use an aqueous electrolytic solution. In recent years, researches particularly on the power source for electric vehicle, in which an internal combustion engine and a lithium secondary battery are hybridized, have been actively carried out in the US, Japan, Europe, and the like.
However, a nickel-hydrogen battery has been used so far from the viewpoint of safety although the use of a lithium ion battery as a large-sized battery for an electric vehicle is considered from the viewpoint of the energy density. The greatest problem to be solved in order to use the lithium ion battery as a power source for electric vehicle is a high price and safety. In particular, the anode active material such as LiCoO2 or LiNdO2 which is currently commercialized and used has a disadvantage that the structure thereof drastically changes when the battery in an overcharged state is heated at from 200 to 270° C., and such a structural change leads to a reaction to release oxygen in the lattice, thus the crystal structure is instable due to the delithiation during charging, and the thermal properties significantly deteriorates.
In order to improve this problem, a part of nickel is substituted with a transition metal element so as to slightly shift the temperature at which the heat generation starts to a higher temperature or to prevent drastic heat generation, and other measures are attempted. The material, LiNi1-xCoxO2 (x=0.1 to 0.3), obtained by substituting a part of nickel with cobalt exhibits excellent charge and discharge characteristics and lifespan characteristics but still has a problem of thermal stability. In addition, a number of technologies related to the composition and production of a Li—Ni—Mn-based composite oxide obtained by substituting a part of Ni with Mn which exhibits excellent thermal stability or a Li—Ni—Mn—Co-based composite oxide obtained by substituting a part of Ni with Mn and Co are known, and a new-concept anode active material has been recently disclosed in Japanese Patent Application. Laid-Open No. 2000-227858 in which not LiNiO2 or LiMnO2 is partially substituted with a transition metal but Mn and Ni compounds are uniformly dispersed in the atomic level to form a solid solution.
According to European Patent 0,918,041 or U.S. Pat. No. 6,040,090 on the composition of a Li—Ni—Mn—Co-based composite oxide obtained by substituting Ni with Mn and Co, LiNi1-xCoxMnyO2 (0<y≤0.3) exhibits improved, thermal stability as compared to an existing material which is composed of only Ni and Co but still has a problem of thermal stability as a Ni-based material.
In order to solve this disadvantage, a patent on a lithium transition metal oxide having a concentration gradient in the metal composition is proposed in Korea Patent Application. No. 10-2005-7007548. However, by this method, the lithium transition metal oxide can be synthesized so as to have different metal compositions in the inner layer and the outer layer but the metal composition is not continuously and gradually changed in the anode active material thus produced. A gradual gradient of metal composition may be achieved through the heat treatment process, but the interfacial resistance generated between the inner layer and the outer layer may acts as a resistance to inhibit the diffusion of lithium in the particles and this may lead to the deterioration in lifespan characteristics. Moreover, the composition of the inner layer is LCO, Co rich NCM, or a NCM-based material having a nickel content of 60% or less, and the total nickel content is also low, and thus it is difficult to realize a high capacity.
In addition, the powder synthesized by this invention has a low tap density since ammonia of a chelating agent is not used therein, and thus this powder is unsuitable to be used as an anode active material for lithium secondary battery required to have a high energy density.